1. Field of the Invention
The invention relates to a method for dividing the bit rate of QPSK signals into at least two sub-channels having band width limited filters in the modulator and the demodulator, by means of splitting the bit current of the QPSK signals.
2. Description of the Related Art A method of the type is known from IEEE 1999, pages 233 to 238, which method indicates orthogonal pulse shapes in three different ways. In the case of the first method, a low-pass having Nyquist flanks is used, which is operated at half the possible bit rate. The pulse that is orthogonal to this is implemented by means of a time shift. The spectra of the two pulses lie in the same frequency range. Use on multi-carrier systems is not provided. The second method that is used also has the result that the spectra lie in the same frequency range. With the third method, a duobinary signal is generated, the pulse responses of which are limited in terms of time, so that the spectra theoretically reach into infinity. In this connection, the second pulse is also time-shifted, which leads to the spectrum that lies in the same frequency range.
Four orthogonal pulse shapes that can be transmitted in the same channel are known from IEEE 1998, pages 63 to 66. These are attributed to the discrete prolate spheroidal sequences.
In the manual by Docker, Peter, “Dateniübertragung”, Volume I, Fundamentals, 2nd edition 1983, Berlin, which appeared in the Springer Verlag, ISBN 3-540-12117-X, the Nyquist conditions in the case of a data transmission method are discussed on pages 110 to 124. On page 118 ff., the partial response method is described. On pages 144-150 of the manual, amplitude modulation with one-sided band transmission and with remaining side band transmission are described.
In the manual by J. Huber: “Trelliscodierung”, which appeared in the Springer Verlag in 1992 under the series Nachrichtentechnik, 21, ISBN 3-540-55792-X, modulation with time-limited signal elements in the coding and modulation of pulses is described on page 12, furthermore digital pulse amplitude modulation, which is also used in the inventive method, is described on page 13 ff.
Modulation systems for QPSK, MSK, SFSK, and DSFSK signals are furthermore known from IEEE Transactions on Communications, Vol. 42, No. 2/3/4, Feb./Mar./Apr. 1994, pages 1465 ff.
Furthermore, the basics of the PSK method are known from the technical manual “Nachrichtentechnik” by E. Herter/W. Lörcher, 5th edition, which appeared in the Hanser Verlag in 1990, pages 110 ff., and the implementation of PSK modulators and demodulators and frequency multiplication were described there. Thus it is possible to generate a carrier 2ƒT from a 2-PSK signal, by means of squaring, from which the desired carrier ƒT results afterwards, by means of frequency division. For this, it is indicated that in general, squaring has to occur in the case of an N-PSK signal ld(n)rnd. During squaring, the phase angles are doubled. After the first squaring step at 2-PSK, the signal therefore is given the phase position 0 and 360°. But since these phase positions are the same, the spectrum of the signal that has been squared twice contains contributions, after the phase angle doubling, which point in the same direction. Seen spectrally, this means that the desired line is reached at a multiple of the original carrier frequency ƒT, for example at four ƒT. The reference carrier of the frequency ƒT that is obtained by means of frequency division, in this connection, has a phase that is displaced by n×π/2 (n=0. . . 3), as compared with the correct zero phase.
From the IEEE Transactions on Communications 37, No. 5 (May 1989), pages 437 to 448, a proposal is known how the bit rate of QSPK can be doubled by adding a second orthogonal signal. FIG. 4 on page 447 shows such signal shapes. Because of the perpendicular flanks of the pulses, the band width is very great, i.e. the orthogonality is lost when the band width is limited, and inter-symbol interference (ISI) and cross-talk (ÜS) occur between the channels. At the end of the essay, the authors, D. Saha and G. Birdsall, discuss systems limited in band width, which use band width limited transmission filters P1 and P2 and corresponding matched filters P1* and P2* on the receiving side (FIG. 13 on page 446). The bit rate 1/T=2fg for one branch of a QPSK system (in other words a total of 4fg) is split into ½T twice there, and is therefore the same as for QPSK. This arrangement is used for the sine carrier and the cosine carrier, in each instance. The authors make the statement that there are infinitely many possibilities for the pairs P1 and P2, and give three examples in FIG. 14 on page 447, without the related pulse responses of the individual filters P1 and P2, the transmission and reception filters P1P1* and P2P2*, and do not discuss the cross-talk P1P2*. Since the filters P1 are real and P2 are imaginary, it holds true that P1*=P1 and P2*=−P2. A closer examination shows that the conditions free of ISI and ÜS can only be achieved with the examples (a) and (b), and that the example (c) according to FIG. 14 does not fulfill the conditions, in disadvantageous manner.
The idea of adding a second pulse, orthogonal to the control pulse of the QPSK, for modulating the sine carrier and the cosine carrier, is also known from U.S. Pat. No. 4,680,777.